As conventional failure detection/switching technologies in packet communications, there are STP (Spanning Tree Protocol) in the Ethernet, derivatives of it (refer to non-patent documents 1-3, for example), and EAPS (Ethernet Automatic Protection Switching). In the EAPS, there is a technology for performing switching at high speed in a ring topology for a MAN service.
In this technology, master switches are selected in the ring so that one of the master switches is selected as a primary switch and another is selected as a secondary switch so as to block the secondary switch. The primary switch sends “Hello packet” over the ring, and a failure is detected if the packet does not return to the secondary switch within a predetermined time. When a switch on the ring detects a failure, the switch can send “TRAP” to the master switch, so that the failure can be detected more quickly (within one second). When the failure is detected, the secondary switch is changed to “FORWARDING” immediately.
In addition, as a standard for realizing high reliability in the ring network, there is RPR (Resilient Packet Ring) (refer to document 4, for example).
Further, as an uninterruptible technology for ATM cells in an ATM line, there is a method for writing a cell number on the ATM cell, copying the cell into equal to or more than two, sending each cell to different lines, and in the receiving side, selecting one of the same information cells sent from equal to or more than two lines to transfer the cell downstream (refer to patent document 1, for example).
[Non-patent document 1] IEEE 802.1D Spanning Tree Protocol (STP)
[Non-patent document 2] IEEE 802.1w Rapid Spanning Tree (RSTP)
[Non-patent document 3] The Switch Book: The Complete Guide to LAN Switching Technology by Rich Seifert, translated by Nikkei Business Publications, Inc., ISBN4-8222-8099-3
[Non-patent document 4] IEEE802.17 Resilient Packet Ring (RPR)
[Patent document 1] Japanese Laid-Open Patent Application No. 7-46250